Recently, a vehicle having an automatic transmission includes an idling stop system that stops an engine of the vehicle when a transmission mode of the automatic transmission is set at a frontward-run mode and when a vehicle speed is equal to or lower than a predetermined value. The automatic transmission controls an oil pressure to control engagement or separation of a friction element by using generally a spool linear solenoid. The linear solenoid includes a spool and a sleeve accommodating the spool slidably, and a clearance between the spool and the sleeve is set lower than several tens of micrometers in order to reduce a leakage of hydraulic oil from between the spool and the sleeve. Thus, if foreign materials contained in the hydraulic oil penetrate into between the spool and the sleeve, reciprocation motion of the linear solenoid may be interrupted, and an operation of the automatic transmission may be affected.
An oil-pressure supply device which supplies an oil pressure to the automatic transmission has a strainer provided in an oil passage connecting an oil reservoir and a suction port of a pump that supplies an oil pressure to the friction element. The strainer traps foreign materials contained in the hydraulic oil. A pore size of a filter element of the strainer may be set at a few micrometers to be capable of trapping foreign materials that are to penetrate into the spool and the sleeve of the linear solenoid. However, in this case, the filter element may be clogged with the foreign materials.
Patent Document 1 (JP 2005-273633 A) discloses a method to prevent clogging of a filter element of a first strainer. An oil-pressure supply device described in Patent Document 1, pumps a hydraulic oil stored in an oil reservoir by using an oil pump and an electric pump to supply the pumped hydraulic oil to parts of an engine. The first strainer is provided in an oil passage connecting the oil reservoir and the electric pump. A second strainer is provided in a branch passage that branches from an oil passage between the oil reservoir and the first strainer, and the branch passage is connected to the oil reservoir. A flow sensor is provided in the oil passage between the oil reservoir and the first strainer to detect a flow rate of the hydraulic oil. When a flow rate of the hydraulic oil flowing through the oil passage between the oil reservoir and the first strainer is equal to or lower than a predetermined flow rate, an electronic control unit: ECU rotates the electric pump inversely to remove foreign materials trapped in the filter element of the first strainer. The removed foreign materials pass through the branch passage to be trapped by the second strainer. Accordingly, the first strainer is recovered.
A strainer used for an automatic transmission generally continues to be used in all life span of the automatic transmission. If the method of the Patent Document 1 is applied to the oil-pressure supply device of the automatic transmission, the foreign materials trapped in the second strainer cannot be removed and the second strainer may be clogged. Furthermore, the oil-pressure supply device of the Patent Document 1 includes the flow sensor and the ECU. Hence, the flow sensor and its wiring may need to be provided, and a volume of the ECU may be increased. As a result, the system may be increased in size, and its manufacturing cost may be enhanced.
An objective of the present disclosure is to provide an oil-pressure supply device that restricts clogging of a filtration portion which traps foreign materials contained in a hydraulic oil supplied to an automatic transmission.